This invention relates to scanning probe microscopes and, more particularly, in a scanning probe microscope having a probe wherein the relationship between the probe and a sample to be scanned is defined by three legs, to the improvement to allow tilt between the probe and the sample to be adjusted comprising, each of the three legs including adjusting means for adjusting a length thereof; and, tilt control means attached to the adjusting means for independently adjusting the length of selected ones of the three legs.
Scanning probe microscopes (SPMs) are instruments that provide high resolution information about the properties of surfaces. One common use of these devices is imaging, and some types of SPM have the capability of imaging individual atoms. Along with images, SPMs can be used to measure a variety of surface properties, over the range from a few angstroms to hundreds of microns. For many applications, SPMs can provide lateral and vertical resolution that is not obtainable from any other type of device.
The first type of SPM developed was the scanning tunneling microscope (STM). The STM places a sharp, conducting tip near a surface. The surface is biased at a potential relative to the tip. When the tip is brought near the surface, a current will flow in the tip due to the tunneling effect. Tunneling will occur between the atom closest to the surface in the tip and the atoms on the surface. This current is a function of the distance between the tip and the surface, and typically the tip has to be within 20 angstroms of the surface for measurable current to be present. An STM has a mechanism to scan the tip over the surface, typically in a raster pattern. While the tip is scanned over the surface, the tip is kept at a constant distance above surface features by means of a feedback loop employing the tunneling current and a vertical position controlling mechanism. The feedback loop adjusts the vertical position of the tip to keep the tunneling current, and thus the distance, constant. The vertical position of the tip is determined from the control signals applied to the vertical position controlling mechanism. The vertical position, as a function of horizontal scan position, produces a topographic map of the surface. STMs can easily image individual atoms, and can also be used for highly accurate surface measurements on larger scales, up to a few hundred microns. STMs also may be used for data other than topographic images. One alternative operation of an STM is to hold the tip stationary while varying the bias voltage applied to the sample and monitoring the tunneling current, thus measuring local current/voltage characteristics of the surface. STMs require a conducting sample surface for operation. Non-conducting surfaces may be coated with a thin conducting material such as gold or, in some cases, non-conducting materials a few atoms thick lying on a conducting surface may be imaged.
Another SPM, the atomic force microscope (AFM), similarly scans a tip across a surface. The tip in this case is mounted on the free end of a lever or cantilever which is fixed at the other end. The tip is brought to a surface such that the force interaction of the tip with the surface causes the cantilever to deflect. An AFM may be operated such that the Van der Waals attractive force between the tip and surface are near equilibrium with the repulsive force, or at larger cantilever deflections where the repulsive force, dominates. A feedback loop employing the cantilever deflection information and the tip vertical position is used to adjust the vertical position of the tip as it is scanned. The feedback loop keeps the deflection, and thus the force, constant. The tip vertical position versus horizontal scan provides the topographic surface map. In this mode, the forces on the surface can be made very small so as not to deform biological molecules. AFMs can also be operated in a mode where the repulsive force deflects the cantilever as it scans the surface. The deflection of the tip as it is scanned provides topographic information about the surface. AFMs may also be operated in a non-contact mode where the cantilever is vibrated and the Van der Waals interaction between the tip and surface affects the vibration amplitude. AFMs have a means to detect the small movements of the cantilever. Several means for cantilever motion detection have been used with the most common method employing reflected light from the cantilever. The deflection of a light beam due to the cantilever motion may be detected, or the movement of the cantilever can be used to generate interference effects which can be used to derive the motion. Like an STM, AFMs can image individual atoms; but unlike an STM, AFMs can be used for non-conducting surfaces. AFMs may also be used for measurements such as surface stiffness.
Other SPMs may use different probing mechanisms to measure properties of surfaces. Probing devices have been developed for such properties as electric field, magnetic field, photon excitation, capacitance, and ionic conductance. Whatever the probing mechanism, most SPMs have common characteristics, typically operating on an interaction between probe and surface that is confined to a very small lateral area and is extremely sensitive to vertical position. Most SPMs possess the ability to position a probe very accurately in three dimensions and use high performance feedback systems to control the motion of the probe relative to the surface.
The positioning and scanning of the probe is usually accomplished with piezoelectric devices. These devices expand or contract when a voltage is applied to them and typically have sensitivities of a few angstroms to hundreds of angstroms per volt. Scanning is implemented in a variety of ways. Some SPMs hold the probe fixed, and attach the sample to the scanning mechanism while others scan the probe. Piezoelectric tubes have been found to be the best scanning mechanism for most applications. These tubes are capable of generating three dimensional scans. They are mechanically very stiff, have good frequency response for fast scans, and are relatively inexpensive to manufacture and assemble. Such scanners are used in a commercial STM sold by the assignee of this application, Digital Instruments, Inc., under the trademark NanoScope. These scanners are made in various lengths, the larger ones having larger scan ranges.
As can be appreciated, SPMs are extremely useful research tools, allowing for information of higher resolution to be obtained more conveniently than previously possible. Some aspects of SPM performance require improvement, however, in order for SPMs to become more practical for applications requiring less operator interaction, accurate repeatable measurements for larger scale samples, and high throughput.
In the scanning probe microscope, the piezoelectric scanners typically have ranges of a few microns, so the sample must be brought close to the probe with some kind of mechanical arrangement in order for the probing of the surface to occur. Presently, these arrangements include moving the sample straight toward the probe with a screw or piezoelectric inchworm, or tilting the scanner support to bring the probe toward the surface. A prior art scanning probe microscope, which is most representative of scanning tunneling microscopes, is illustrated in FIG. 1 where it is generally indicated as 10. In this device, a scanner 12 rests on two fixed supports 14 and one movable support 16 attached to a base 18. The fixed supports 14 can be hand adjusted while the movable support 16 is motor driven and allows for automatic final approach. The scanner 12 must be hand adjusted and leveled; so, the probe 20 must be placed very near the sample 22 by eye, usually using an optical microscope, before the automatic approach is engaged. This procedure is not difficult; but, requires an operator to prepare each new probe site by hand. Other prior art SPMs utilize systems that translate the scanner toward the sample with a motion parallel to its axis. These systems may be operated with less operator participation; but, have no flexibility to adjust for sample tilt.
In many instances and for several reason, it would be useful to have the ability to control the tilt of the scanner with respect to the sample independent of positioning the probe vertically. One reason is related to the errors caused by non-linear behavior of the piezoelectric scanning elements. Piezoelectric non-linearity is a well known source of error in the art, and can affect SPM data in many ways. For large scans, one non-linear error is related to tilt between the probe and the sample. It is extremely difficult to mount a sample such that, on the scale of SPM measurements, there is not some tilt between the sample and probe. For large scans, the cumulative non-linearity errors due to the scanner make a tilted flat surface appear bowed. As one useful application of SPMs for larger scale samples is surface dimensional measurements, the distortion of a tilted sample is a serious problem. The tilt may be on only part of the sample, so having a flat sample holder will not solve this problem. What is needed is a scanner which minimizes this distortion by having the scanner able to be tilted with respect to the sample, thereby allowing compensation for an effect that otherwise decreases the utility of the instrument.
On the other hand, in the scanning of surfaces which have very steep features, such as the surface of an integrated circuit, it is useful to have a known tilt between the probe and sample. Given a tapered probe 20, such as an etched tungsten probe in the case of an STM, the probe 20 will have some angle for its profile, as indicated by the arrows in FIG. 2. If the probe 20 is perpendicular to the bottom of a groove 23 as depicted in that figure, it can be seen that it is impossible to scan all the way to the edge of the groove 23 as the side of the probe 20 will hit the side of the groove 23 before the scanning point of the tip. Thus, in order to scan to the edge of the groove 23, one must tilt the scanner (and therefore the probe 20) with respect to the sample 22 by an angle which is greater than the tip profile angle as depicted in FIG. 3. A lesser tilt would, of course, improve the situation but not completely solve it. As shown, the tilting allows the tip of the probe 20 to travel down the sidewall and determine its profile. The scanner and probe 20 would be tilted in the opposite direction in order to image the other side of the groove 23. The images of the tilted surfaces could then be patched together with the computer to construct a proper image reflecting the true surface topology of the entire groove 23. A similar procedure could be used for any very steep feature, such as a step or bump. As will be seen, this unique method is possible with the present invention as described hereinafter.
Not only would it be desirable to be able to tilt the scanner with respect to the sample in a controlled manner in order to remove tilt or create known tilts; but, it would be desirable also to be able to automatically approach the sample with the scanner in a straight line fashion over a long range so that there is no need to manually place the tip near the surface with a microscope or magnifier. Most desirable would be to have both of these abilities in a single device as it is not practical to approach a new sample or a new sample section automatically without some means to adjust the tilt. These abilities along with the ability to translate a large sample underneath the probe, or the ability to automatically sequence a series of samples to the probe would allow SPMs to be used for totally automatic inspection and characterization of either large area samples or multiple samples. Such capabilities would make SPMs much more useful for industrial applications such as imaging magnetic disks or integrated circuit wafers.
Wherefore, it is an object of this invention to provide a scanning probe microscope head which has both vertical motion and tilt motion.
It is another object of this invention to provide a scanning probe microscope head which can be used conveniently in SPMs that will have the capability for large samples, fully automated operation, and multiple samples.
Other objects and benefits of the invention will become apparent from the detailed description which follows hereinafter when taken in conjunction with the drawing figures which accompany it.